1. Field of the Invention
The present invention relates to a technology for determining the state of a paper sheet such as a paper currency.
2. Description of the Related Art
In a paper sheet processor for processing a paper sheet such as a paper currency, conventionally, a technology for dividing the whole paper sheet into infinitesimal areas first, and for utilizing an electrical signal of each infinitesimal area obtained from an optical sensor or a thickness sensor etc. is used for measuring the shading of the pattern, the shape and other characteristics of the paper sheets. The sensor-measured value obtained by such a technology is converted into a gradation signal and then is stored. A conversion into a gradation signal with 256 gradations is applied, for example. The authenticity and the extent of damage are determined by performing image processing to the gradation signal and by comparing the data after image processing with pre-prepared template data (template).
The template data comprises values, which are considered a reference in various determinations, such as a set of data to which the gradation is set. Here, the template data used in the above determinations is prepared to be of the same number as the number of combinations of types, sides (both) and the feed directions that the paper sheet is fed into the device. In other words, for one type of paper currency, (a surface of the side read by a sensor: two combinations from two sides).times.(the direction fed into the device: two combinations from forward and backward directions)=4 combinations of the template data is prepared.
In the above paper sheet processor, in order to facilitate the comparison of the template data with the data obtained from an actual paper sheet, the obtained data may be corrected. When, for example, the average gradation of a paper sheet is smaller than that of the template data due to a contamination on the paper sheet, the value of each infinitesimal area is corrected so that the average value reaches the same brightness as that of the template data.
FIG. 1 is a diagram explaining a conventional data correction method. An explanation of the correction method is provided using specific numeric values with reference to FIG. 1. Assume that the average color density of the template data, which is the reference, is “100” in 256 gradation levels for simplicity in the following explanation.
First, a case that a paper sheet with darker color as a whole due to contaminations etc. is fed into the paper sheet processor is examined. In this case, suppose that actual data 61 is obtained from the fed paper sheet. The average gradation of the actual data 61 is “80”, which is lower than the reference template data. In order to compare the actual data 61 with the template data 63, in which the average gradation is “100”, corrected data 62 is obtained by multiplying the actual data 61 by a coefficient “100/80”. The average gradation of the corrected data 62 is 80.times.(100/80)=100. For each infinitesimal area, the corrected data 62 and the template data 63 are compared, and the authenticity etc. of the paper sheet determined.
Next, a case that a paper sheet, which is darker in part due to contaminations etc., is fed is examined. Suppose that actual data 64 is data obtained from a paper sheet of which about ½ of the whole paper sheet area is contaminated. Within the actual data 64, an area A has the average gradation of “20” due to heavy contamination, and an area B with its average gradation of “80” has less contamination than the area A.
For actual data 64 associated with such conditions, the average density of the actual data 64 is (20+80)/2=50. At that time, in the same way as the above procedure, corrected data 65 corrected from the actual data 64 has an area A with its gradation of 20.times.(100/50)=40 and an area B with its gradation of 80.times.(100/50)=160. Using the corrected data 65, comparison with the above template data 63 is performed.
In addition to the above method, as described in Japanese laid-open Patent Application Publication No. S59-160284, Japanese laid-open Patent Application Publication No. 2000-182115 and Japanese laid-open Patent Application Publication No. S53-100895, there is another method in which image processing is performed on a prescribed area of paper sheet image data, and the state of the paper sheet is determined. In such a method, adjustments such as weighting of a part where a watermark etc. is arranged on a paper sheet or gain of the output signal of a scanner scanning the part is performed.
The above method, which corrects the average gradation of a whole paper sheet so that the average gradation of the whole paper sheet corresponds to that of the reference template data (“100” in above example), corrects the whole uniformly even in a case that the actual data 64 in FIG. 1 in which an area A where the gradation is extremely low is partially present. For that reason, the relatively bright area B has an excessively high gradation compared with the template data because of the correction, and the authenticity etc. of the paper sheet may not be determined correctly.
In the case that a paper sheet is contaminated over a wide area as in the actual data 61 in FIG. 1, when the average gradation is corrected to “100”, that the entire paper sheet is contaminated may not be recognized. In other words, the paper sheet is a contaminated paper sheet to be collected by the device under normal conditions; however the paper sheet is not recovered, and is to be withdrawn by a user etc. of the device.